Function measure_cue 

pub fn measure_cue(
    show: &str,
    episode: &str,
    cue: &str,
    effect: &Map<String, Value>,
    duration_fn: &'_ dyn Fn(&Path) -> Result<i64, String>,
    workspace: &Path,
) -> Option<CueImpact>

Measure one entry; None when it has no source.